Glaciers, lakes, rivers, and vegetation are particularly sensitive to the change of precipitation in the transition zone between arid and humid regions of the Tibetan Plateau. This study analyzed the spatiotemporal evolution of precipitation and its mechanisms under dynamic and thermodynamic processes based on in situ and satellite precipitation data of the Tibetan Plateau. The results indicate that the proportion of areas with annual precipitation <400 mm or >2000 mm decreases, while the proportion of areas with annual precipitation between 400 and 2000 mm increases from 1998 to 2018. This shows that precipitation in arid, semi-arid, and semi-humid regions exhibits an increasing trend, while that in southeastern humid regions exhibits a decreasing trend. Moreover, the dynamic process has a stronger effect on the change of precipitation than the thermodynamic one. In particular, the former is twice as strong as the latter, indicating that the atmospheric circulation wind has a more significant impact on the precipitation change. Although the arid, semi-arid, and semi-humid regions show a wetting trend, their physical mechanisms are different. The decrease of negative contribution of the advection and the increase of positive contribution of the convergence lead to the increase of precipitation in northwestern arid and semi-arid regions, while the convergence and advection show a slight decrease in the eastern semihumid and humid regions.